The present invention relates to power supply systems and particularly to such systems including a generator producing a DC output, a battery charged by the generator and supplying the power in case of failure or termination of operation of the generator, and generator regulating means regulating the output voltage of the generator. In aircraft power supply systems, the batteries may also be used for starting the aircraft and/or for providing power for auxiliary units.
Many batteries, particularly nickel-cadmium batteries, when connected to a bus bar at a constant potential, may overcharge such as to produce thermal runaway. This can result not only in the destruction of the battery, but also in an explosion. For this reason, the charge rate of the battery should be closely controlled particularly when the battery is used in an aircraft power supply system.
Some aircraft power supply systems do not include automatic control to prevent battery over-charging, but rather include generator control units which regulate the output of the generator to maintain a constant potential, the batteries being connectable to the buses by cockpit switches. Such systems however, suffer from the risk of thermal runaway because of battery over-charging, as noted above.
Conventional systems for preventing battery over-charging (e.g., of nickel-cadmium batteries) on aircraft usually include a battery charger for each battery. The charger is supplied from the aircraft bus and charges its battery according to a predetermined program, usually at a constant current until the battery attains a certain value indicating about 100% capacity, and then preserves this charge condition. Such aircraft systems also include a battery-line-contactor controlled by the battery charger, which contactor automatically connects the battery to the bus whenever the voltage on the bus drops below a certain level, or whenever a failure or termination of operation of the generator is experienced. Although such battery-charger systems have the advantage that they maintain the battery at about 100% capacity, they have a number of disadvantages. Thus, a special charger is required for each battery, which involves added weight and relatively high additional cost; in addition, the battery is neutralised from the bus bar and therefore does not participate in the transient phenomena nor serve as a filter, the result being a poor quality voltage on the buses; further a continuous voltage supply on the bus bars is not guaranteed, which can produce a loss of memory in data processing systems having a volatile memory; still further, a disconnect and connect logic between the battery and the bus bar is required, which can impair system reliability especially in the case of an emergency when power might be needed.